Machine control, especially that of induction motors, is an essential consideration on the program of any industrial world. Efficient machine control, not only saves power consumption, but increases productivity. Threephase induction motors produce mechanical power by electromagnetic induction and run on a 3-phase ac supply. They require efficient speed control, to enable them do variable speed operations, save power consumption and reduce machine noise. In this research work, a new switching device called Mos-Controlled Thyristor (MCT) for frequency drive is introduced. Based on the new switching device and AT89C52 microcontroller, an enhanced frequency drive for controlling the speed and torque of 3-phase 15kW squirrel cage induction motor is modeled. Different voltages ranging from 342V to 415V and frequencies ranging from 50Hz to 60Hz are used in a systematic manner to simulate the system based on the new switching device. The simulation program is written in C language and tested with Proteus 7.6 simulation software. Voltage and frequency have significant impact on the actual speed and torque of the motor. Simulation results show that with the new model, the torque (56.66Nm) developed by the motor which is constant throughout each speed range is directly proportional to the ratio (6.7) of the applied voltage and the frequency of the supply and the selected speeds (1450, 1510, 1570, 1630, 1690 and 1750 rpm) are locked irrespective of change in load. This is unlike other models where magnetic saturation and conduction drop of IGBT lead to voltage/frequency imbalance resulting in excessive drawing of current by the motor and overheating. This new control method has a speed regulation of ±2 to 3% of maximum frequency, speed response of 3Hz, speed control range of 1: 40 and efficiency of 88%, as further advantages. Comparison of the system with other speed control techniques shows improved energy-saving, cost effectiveness and safety in operation.
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